1. Field of the Invention
This invention relates to implantable medical devices, such as stents. In particular, the invention relates to bioabsorbable polymeric stents with a radiopaque layer.
2. Description of the State of the Art
This invention relates to radially expandable endoprostheses, which are adapted to be implanted in a bodily lumen. An “endoprosthesis” corresponds to an artificial device that is placed inside the body. A “lumen” refers to a cavity of a tubular organ such as a blood vessel. A stent is an example of such an endoprosthesis. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The structure of stents is typically composed of scaffolding that includes a pattern or network of interconnecting structural elements or struts. The scaffolding can be formed from wires, tubes, or sheets of material rolled into a cylindrical shape. In addition, a medicated stent may be fabricated by coating the surface of either a metallic or polymeric scaffolding with a polymeric carrier. The polymeric scaffolding may also serve as a carrier of an active agent or drug.
The first step in treatment of a diseased site with a stent is locating a region that may require treatment such as a suspected lesion in a vessel, typically by obtaining an x-ray image of the vessel. To obtain an image, a contrast agent, which contains a radiopaque substance such as iodine is injected into a vessel. “Radiopacity” refers to the ability of a substance to absorb x-rays. The x-ray image depicts the lumen of the vessel from which a physician can identify a potential treatment region. The treatment then involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through a bodily lumen to a region in a vessel that requires treatment. “Deployment” corresponds to the expanding of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into a bodily lumen, advancing the catheter in the bodily lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen. In the case of a balloon expandable stent, the stent is mounted about a balloon disposed on the catheter. Mounting the stent typically involves compressing or crimping the stent onto the balloon. The stent is then expanded by inflating the balloon. The balloon may then be deflated and the catheter withdrawn. In the case of a self-expanding stent, the stent may be secured to the catheter via a retractable sheath or a sock. When the stent is in a desired bodily location, the sheath may be withdrawn allowing the stent to self-expand.
The stent must be able to simultaneously satisfy a number of mechanical requirements. A stent must be capable of maintaining a desired degree of patency of a vessel. To accomplish this, the stent must be capable of withstanding the structural loads, namely radial compressive forces, imposed on the stent as it supports the walls of a vessel lumen. In addition to having adequate radial strength or hoop strength, the stent should be longitudinally flexible to allow it to be maneuvered through a tortuous vascular path and to enable it to conform to a deployment site that may not be linear or may be subject to flexure. The material from which the stent is constructed must allow the stent to undergo expansion, which typically requires substantial deformation of localized portions of the stent's structure. Once expanded, the stent must maintain its size and shape throughout its service life despite the various forces that may come to bear thereon, including the cyclic loading induced by the beating heart. Finally, the stent must be biocompatible so as not to trigger any adverse vascular responses.
In addition to meeting the mechanical requirements described above, it is desirable for a stent to be sufficiently radiopaque to be fluoroscopically visible under x-rays. Accurate stent placement is facilitated by real time visualization of the delivery of a stent. A cardiologist or interventional radiologist can track the delivery catheter through the patient's vasculature and precisely place the stent at the site of a lesion. This is typically accomplished by fluoroscopy or similar x-ray visualization procedures. For a stent to be fluoroscopically visible it must be more absorptive of x-rays than the surrounding tissue. Radiopaque materials in a stent may allow for its direct visualization.
It may be desirable for a stent to be biodegradable. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Therefore, stents fabricated from bioerodable materials such as biodegradable polymers can be configured to completely erode only after the clinical need for them has ended.
However, all commercially available bioabsorbable polymers are radiolucent and do not possess sufficient radiographic density to be easily imaged by fluoroscopy. Biodegradable polymers tend to have x-ray absorption properties similar to body tissue.